The present invention pertains to the signal processing art and, more particularly, to an improved squelch detector.
Squelch circuits are well known, especially in the communications art. Communication receivers utilize squelch systems to mute the receiver audio during the absence of a received signal. Without a squelch circuit, the receiver would be continuously producing annoying random noise audio during the intermission between broadcast signals.
Most squelch systems employ a squelch detector, which is a time averaging circuit that opens the audio channel once a broadcast signal is detected. Since squelch detectors are basically integrating type systems, there is a distinct time lag from the time a broadcast signal is received for turn on of the audio channel and, similarly, a time lag following the received signal before the audio channel is muted.
As a result of the detector time lag at the instance of reception of a received signal the initial portion of the information signal may be lost, thus reducing intelligibility of the overall communication system. During the time lag following the termination of a broadcast signal, when the squelch detector maintains the audio channel open the receiver operator is subjected to the above mentioned annoying random noise.
Considerable effort has been spent by those in the communication art to minimize or eliminate the above described failings of conventional squelch systems. In one approach, special circuitry is provided within the transmitter to precede and succeed each information signal broadcast by either a predetermined period dead carrier time or a coded tone. Corresponding unique circuitry is provided in the receiver to delay and process the received signal such that the squelch opens the audio channel prior to the actual information message and closes the channel prior to the end of the total transmission. This approach, while possibly effective in eliminating the annoyance of delay in the squelch detector, is impractical in widespread operation. For example, practical implementation of the prior art system would require strict standardization of all transmitters and receivers such that they contain the appropriate encoding and decoding circuitry. This would necessarily result in a substantial increase of overall system cost.